Seismic detectors are used to detect earth motion caused by natural phenomena such as earthquakes, microseisms and landslides and man made phenomena such as chemical or nuclear explosions. A seismograph is a complete instrument for detecting and recording earth motions and consists of a seismic detector, and a recording means including a chronograph for indicating the time of any detected motion.
Most seismic detectors, including the instant invention, rely on the inertia of a mass which is relatively free to move in at least one direction relative to the ground. As the ground moves during an earthquake the restoring force on the mass is sufficiently weak so it does not follow the earth's motions, and a sensing means can measure the relative motion of the earth and the mass. Mechanical seismographs couple this relative motion directly to drive a pen which writes upon a revolving drum to record the earth motion. Opto-mechanical seismographs use a light beam reflected from a mirror coupled to the stationary weight to directly record the motion upon photographic medium attached to a revolving drum. Electromechanical types, e.g. the Galitzin, uses a coil attached to the mass and a permanent magnet attached to a frame fixed to the earth. During the earthquake the magnet moves within the coil thereby inducing a voltage within the coil. This voltage can then drive a galvanometer or other recording means. The Galitzin sensor mentioned above uses the linear motion of the coil and the magnet or magnetic circuit to cause a change in the flux through the coil. Other sensors use other electrical and physical properties of materials to detect the relative motion of the inertial mass and the earth. One advantage of electromechanical types of seismic detectors is that the voltage they produced can be processed by modern electronic circuits including computers thereby amplifying, filtering, calibrating, storing and displaying the signals. In addition the signals from a number of sensors can be compared and analyzed to provide additional information about the source of the earth's motion.